For many years, air conditioning and/or refrigeration systems (hereinafter collectively referred to as "refrigeration systems" or "air conditioning systems") operating on the vapor compression cycle and employed in vehicular applications utilized rather bulky and inefficient heat exchangers for both the system condenser and the system evaporator. For example, condensers were typically of the serpentine type having a single or occasionally two passes. In order to avoid excessive refrigerant side pressure drops because of the lengths of each run, the refrigerant confining tubing, typically a multi-passage extrusion, had a relatively large tube minor dimension. For any given facial area occupied by the core of the condenser, the relatively large tube minor dimension reduced the air free flow area through the core, thereby inhibiting heat transfer.
Refrigeration system evaporators were generally of three differing types. One type also was a serpentine tube construction using an extruded tube having a tube major dimension that typically was on the order of four inches. The resulting evaporator cores were relatively deep and as a result, air side pressure drop across the evaporator was relatively high and that in turn reduced the amount of air that could be forced through the evaporator and/or required a larger fan and more energy to drive it. The relatively large tube minor dimension of the tubes used in these constructions also affected air side pressure drop adversely, exacerbating the problem. Furthermore, with such a core depth, draining of condensate from the core was difficult. As a result, condensate from the ambient air would further increase the air side pressure drop. In addition, the film of water forming on evaporator parts impeded heat transfer.
Still another type of evaporator more typically found in home refrigeration units as well as in vehicles was a so called round tube plate fin evaporator. These constructions were relatively bulky and because round tubes were utilized, the air side free flow area through the core was decreased considerably, adding to inefficiency of the unit.
Some of these difficulties were cured by resort to so called "drawn cup" evaporators. However, drawn cup evaporators still required a typical core depth of three inches and large minor dimension tubes, and as a consequence, air side pressure drop remained relatively high as did the inefficiencies associated therewith.
In the mid 1980's, so called "parallel flow" condensers began to reach the market for use in automotive air conditioning systems. A typical parallel flow condenser is illustrated in the U.S. Pat. No. 4,998,580 to Guntly and assigned to the same assignee as the instant application. Parallel flow condensers utilize relatively small header and tank constructions that were highly pressure resistant and which had a plurality of flattened tubes extending between parallel headers. The flattened tubes could be either extruded or fabricated with inserts. In either event, each tube had several flow paths extending along the length thereof, each of which were of a relatively small hydraulic diameter, that is, up to about 0.07". Hydraulic diameter is as conventionally defined, that is, four times the cross-sectional area of each flow path divided by the wetted perimeter of that flow path.
Substantial increases in efficiency were immediately noted. Excellent heat transfer was obtained with units that occupied a significantly lesser volume than prior art condensers and which weighed substantially less.
It was surmised that these and other efficiencies might also be obtainable in parallel flow evaporators.
Consequently, work was performed on utilizing parallel flow type constructions with tubes having flow paths of relatively small hydraulic diameter. An example is shown in commonly assigned Hughes Pat. No. 4,829,780, issued May 16, 1989.
This patent recognizes that whereas an efficient parallel flow condenser can be achieved using a single tube row core, to obtain a high efficiency evaporator, multiple tube rows may be required. It has also been determined that the multiple tube rows should be connected to provide a multi-pass arrangement such that the refrigerant passes two or more times across the path of air flow through the evaporator. As taught by Hughes in commonly assigned U.S. Pat. No. 5,205,347, issued Apr. 27, 1993, a counter-cross flow refrigerant flow is highly desirable. In an example of one such evaporator, two tube rows are employed. In the direction of air flow through the resulting core, refrigerant is inleted to the downstream most one of the tube rows to flow therethrough. After that is accomplished, the refrigerant is directed by a cross-over passage to the forward most one of the tube rows and then once again passed across the path of ambient air travel to be outleted.
These evaporators have worked very well for their intended purpose. For a given frontal area, the same heat transfer can be obtained with a far lesser air side pressure drop in a parallel flow evaporator than in either a serpentine evaporator or a drawn cup evaporator. Furthermore, when intended for use in vehicular air conditioning systems, a parallel flow evaporator has a decided advantage because of its low volume. As is well known, an air conditioning evaporator in an automobile is typically housed under the dash. With increasing emphasis on equipping automobiles with air bags, under dash space is at a premium. A typical parallel flow evaporator with the same efficiency as a drawn cup or serpentine evaporator and having the same frontal area can be made with a core depth of about two inches whereas a typical serpentine evaporator would require a four inch core depth and a drawn cup evaporator would require a three inch core depth.
Not only does the parallel flow evaporator drastically reduce the volume required, leaving more space under the dash available for other equipment, the far lesser core depth translates to lesser air side pressure drop and increased efficiency either in terms of being able to have a given fan transfer more air through the core to provide greater efficiency, or in allowing a smaller fan to be used, thereby reducing energy requirements for the fan, or both.
Moreover, the lesser core depth of a parallel flow evaporator facilitates better drainage of condensate, thereby promoting efficiency on that score as well.
The lesser volume translates to lesser weight which is an advantage as far as vehicle fuel economy is concerned. It also translates to a lesser material cost, thereby providing a cost advantage over conventional evaporators.
While the evaporators of the Hughes patents identified above have been very successful, they are not without their faults. For example, distribution of refrigerant in an evaporator is extremely important if maximum efficiency is to be obtained. Consequently, distributors are utilized on the inlet side. One such distributor is shown in the previously identified Hughes No. '347 patent and works well for its intended purpose. However, because it is a threaded fitting and basically requires machining of its internal passages, it is an expensive component that greatly adds to the cost of the evaporator.
Furthermore, refrigerant distribution in a cross over between the first and the second pass of the core is of substantial significance as well.
Also of importance is assuring that the incoming stream of refrigerant is uniform at the time it is delivered to the distributor. In a typical case, the refrigerant has already passed through an expansion valve or a capillary and is at a reduced pressure, and therefore, boiling. If uniformity in the incoming stream is not maintained at this time, the liquid refrigerant may tend to separate from the gaseous refrigerant and maldistribution, with accompanying inefficiency, will result.
Finally, it is highly desirable that such an evaporator be relatively simply made with a minimal number of parts so as to be of extremely economical construction to facilitate wide spread use thereof.
The present invention is directed to achieving one or more of the above objects and/or overcoming one or more of the above problems.